1. Field of the Invention
This invention relates to initiation signal transmission lines used in mining and other blasting operations and, in particular, to tubular initiation signal transmission lines such as shock tube and low velocity signal tube.
2. Related Art
U.S. Pat. No. 5,681,904 entitled “Azido Polymers Having Improved Burn Rate”, issued Oct. 28, 1997 and relates to azido polymers, especially cross-linked azido polymers that can be used as high-energy materials. As disclosed starting at column 1, line 10, azido-containing compounds and polymers are important in the fields of explosives and propellants because the azido group is highly energetic and can easily be incorporated into a polymer or oligomer at high weight percent loadings. One especially useful class is described starting at column 1, line 14, as azido-substituted polyethers, for example, glycidyl azide polymer. Although hydroxylated azido-substituted polyethers are often cured with polyisocyanates via a urethane-forming mechanism for energetic material applications, as disclosed starting at column 2, line 9, it has been discovered that liquid azido polymers can be cross-linked to some or all of the azido groups with a multi-functional dipolarophile having a reactive group selected from acrylic and acetylenic esters or amides to produce a polymer material containing triazoline and/or triazole groups. These materials are said to have advantages relative to the polyisocyanate-cured polymers. Such polymers, including glycidyl azide polymers (“GAP”) are commercially available from Minnesota Mining and Manufacturing Company (“3M Company”) of St. Paul, Minn. The disclosure of U.S. Pat. No. 5,681,904 is incorporated by reference herein.
It is conventional practice in mining and other blasting operations to employ non-electric initiation signal transmission tubes to transmit initiation signals from an igniter device to an initiator device such as a detonator that is used to initiate another reactive device, e.g., to set off an explosive charge such as a borehole explosive charge, e.g., a PETN-containing booster charge which, in turn, may initiate a borehole blasting agent such as ANFO. Two well-known types of non-electric signal transmission tubes are known in the art as shock tube and low velocity signal transmission tube, and are referred to collectively as signal transmission tubes. Typically, a signal transmission tube comprises a flexible but resilient tube having a thin layer of reactive powder material adhered to the inner wall, leaving a continuous open channel along the length of the tube.
Generally, signal transmission tube may be formed from an extruded synthetic polymeric material such as EAA (ethylene/acrylic acid copolymer), EVA (ethylene vinyl acetate) or a SURLYN™ such as SURLYN™ 8940, an ionomer resin available from E. I. DuPont de Nemours Company of Wilmington, Del., low density polyethylene (LDPE), linear low or medium density polyethylene, linear low, medium and high density polyester and polyvinylidene chloride (PVC), and suitable blends or polymer alloys of such materials. A signal transmission tube may comprise multiple, concentric, co-extruded layers, the outer layer or layers usually being made of a mechanically tougher polymer than the innermost layer. The material used to manufacture the signal transmission tube is generally chosen so that the finished tube will be sufficiently flexible to permit the necessary handling, but will also be of sufficiently high tensile strength and resiliency to resist breakage and sufficiently tough to resist abrasion, cutting or nicking of the tube during use. In fact, conventional signal transmission tubes are so resilient and strong that an initiation signal passing therethrough does not substantially affect the physical integrity of the tube, which remains intact after the signal passes there-through. This allows signal transmission tubes to be used advantageously on the surface of a blasting site where air blast and associated noises are unwanted, as well as for the transfer of an initiation signal through explosive material (such as a borehole charge) to a detonator for the explosive material without causing premature detonation or disrupting the explosive charge in the borehole.
U.S. Pat. No. 5,597,973 to Gladden et al, dated Jan. 28, 1997, entitled “Signal Transmission Fuse”, is concerned with shock tube of specific and inventive dimensions and proportions, and which contains a pulverulent reactive material disposed on the inner surface of the tube. For example, see column 2, line 38 et seq of U.S. Pat. No. 5,597,973.
Another of many patents dealing with shock tube is U.S. Pat. No. 6,170,398 to Rabotinsky et al, dated Jan. 9, 2001, entitled “Signal Transmission Fuse”, which discloses a shock tube which encases a support tape which has a reactive coating adhered to one side of the tape by a binder.
In most cases in the prior art, the reactive material is a pulverulent material which adheres to the interior of the hollow tube by the attraction of the powder particles to the plastic from which the interior wall of the tube is made. That material is usually an ionic ethylene methacrylic acid polymer, such as that sold under the trademark SURLYN® by E. I. DuPont de Nemours Company of Wilmington, Del. The pulverulent reactive material is mainly “unembedded”, meaning that it is not held on the tube wall by an adhesive, binder or the like.
One art-recognized difficulty is migration of the unembedded pulverulent reactive material, which is conventionally held in place only by electrostatic or other attraction to the plastic of which the interior surface of the tube is made. During shipment, handling or installation, portions of the pulverulent reactive material tend to detach from the tube wall, possibly resulting in bare spots on the interior of the tube and/or accumulation of powder, especially in kinks or in curved portions of the tube, which then may be plugged with the loose reactive powder that may interrupt the transmission of a signal therethrough, resulting in a misfire. Powder migration is a problem because, in products where lengths of the signal transmission fuse are connected to devices such as detonators, migrating powder can collect atop the explosive or pyrotechnic contained within the detonator and shield the explosive or pyrotechnic from the signal generated in the shock tube, thereby resulting in a misfire. Localized concentrations of powder can lead to blow-outs of the tube wall which will result in undesired variations of the reaction pressure. Of course, if powder migration is so severe as to leave sections of the fuse with insufficient powder adhered thereto to sustain the reaction, a propagation failure will occur. Reliability of performance of shock tube is always of vital importance, especially in certain applications, e.g., air bag devices, where malfunctioning can lead to injuries.
U.S. Pat. No. 4,756,250 to Dias dos Santos, dated Jul. 12, 1988, entitled “Non-Electric and Non-Explosive Time Delay Fuse, discloses fuses comprising hollow tubes into which pyrotechnic mixtures are blown to deposit pyrotechnic material into the tubes.
Adhering the reactive material to a tape contained within the tube by means of a binder as disclosed in U.S. Pat. No. 6,170,398 is an attempt to overcome the problem of powder migration, but requires a more complicated manufacturing technique.
One disadvantageous result of the resilience, toughness and tensile strength of conventional signal transmission tube such as shock tube is that after the blasting operation, the blasting area is littered with spent but intact tube carcass. The tube carcass may clog up mine processing equipment and may tangle in rotating parts of mining equipment such as the axles or shafts in earth-moving equipment and crushing machinery employed at the blasting site shortly after the tube is used, and may require frequent removal. For example, tube carcasses often snag on earth-moving equipment such as bulldozers, forcing the operator to stop the bulldozer to cut tube carcass from the equipment and to collect and remove tube carcass from the work site. Prior attempts to address this problem have included providing tube that splits upon functioning. On a longer time frame, those portions of conventional tube carcasses, or fragments thereof, that remain on the blasting site or that are transported elsewhere constitute solid waste that is not very susceptible to biodegradation.